In diesel engine applications, it is not uncommon to attempt to attain very high fuel injection pressures even exceeding 1000 bars in order to improve fuel dispersion and to reduce the formation of exhaust emission pollutants. Generally, in such situations a characteristically steep injection curve at the beginning of the injection and a sharply delimited injection end are stipulated. Further, the start and duration of the injection must be adapted to the conditions of the engine performance characteristics. Generally, such adaptation to the engine characteristics is easily accomplished with the employment of associated electronic controls.
Heretofore, purely mechanical injection systems were almost exclusively employed for high pressure injection. Such injection systems always consist of a pump element or system, the injection nozzle and the fluid conduit means interconnecting the pump element and the nozzle. During and after the injection process, strong pressure waves are reflected between the pump and nozzle and the magnitude of such pressure waves may be as much as several-hundred bars. At the pressure waves, in particular after the injection nozzle has closed, zero line contacts may occur at which the vapor pressure of the fuel is fallen short of and this leads to cavitation at the elements of the injection system and to cavity formation with strong shock-like stresses.
In order to obtain a rapid pressure reduction toward the end of the injection, pressure valves at the injection pump are usually provided with relief pistons which increase the volume available to the fuel in the line by the displacement volume. However, it is not always possible to sufficiently reduce the amplitude of the pressure wave reflected during closing with the result that then the reflected pressure wave triggers a new opening process of the nozzle needle valve. It is then that the feared secondary spraying, delayed by the transit time of the pressure wave, occurs resulting in the sprayed fuel being insufficiently atomized and therefore does not completely participate in the combustion.
In injection pumps, the pumping process is fixedly coupled as to a specific angle of engine crankshaft rotation. This results in a high shock-like mechanical load on the injection pump, as the entire pressure buildup takes place within a small angle of rotation in a very short time. As the time for traversing this angle becomes shorter with increasing engine speed, whereas the cross-section of the nozzle holes remain constant, the injection pressure should really increase quadratically with the speed. Fortunately, however, this sharp pressure rise is in large part absorbed by the elasticity of the fuel and of the fuel line or conduit.
Nevertheless, this speed-dependent or related pressure rise leads to considerable problems in the fuel processing or metering. For example, at low speeds the pressure is usually not sufficient to lift the nozzle needle valve completely and because of fuel accumulation or storage in the pressure chamber of the nozzle the pressure rise at the beginning of the injection is further diminished. With the needle valve partially open, the predominant part of the fuel pressure in the valve seat is then transformed into velocity and subsequently swirled in the blind hole of the nozzle. Because of such velocity transformation only a slight fuel pressure is available in front of the nozzle holes, so that a very deficient atomization results. These problems can, of course, be reduced with pintle-type nozzles. Additional secondary spraying occurs also due to the always existing needle valve chatter or bounce when the needle valve sets down in the needle seat.
The strong speed-dependent or related pressure differences make it difficult to adapt the injection nozzle to the requirements of the engine, so that optimum conditions are generally obtained only in narrowly limited engine speed and load ranges.
Furthermore, transit time delays in the fuel lines occur, due to the transport of the pump energy through pressure waves and such makes it difficult to adapt the moment of injection to the requirements of the engine characteristics. In the case of large engines, these problems are no longer controllable because of the relatively long fuel lines. Here, therefore, complicated pump nozzles are required where the pump and nozzle form a unit which is disposed directly in the cylinder head.
For better adaptation of the usual mechanical injection systems to the requirements of the engine characteristics, indirect electronic control of the injection quantity and injection moment is pursued as generally disclosed in Federal Republic of Germany publication DE OS 3024424 A 1. At the individual nozzles, inductive pickups are applied to determine injection start and injection duration. The signals of the inductive pickups and additional operational parameters of the engine are received by an electronic control unit, which in conjunction with a servo magnet adjusts the conventional mechanical injection pump. The injection process, however, which is deficient in broad ranges, cannot be influenced with such a system.
To circumvent the problems resulting from the pressure wave transport of the fuel and which cause most of the difficulties in the usual mechanical injection systems, injection valves may be used where the valve needle is electromagnetically actuated directly. In such an arrangement the pressure chamber of the injection valve is pressurized with a constant fuel pressure so that much smaller pressure fluctuations result upon actuation of the valve and have little influence on the stroke of the needle and valve. There are, however, enormous difficulties in designing sufficiently rapid electromagnets which are able to overcome the high hydraulic forces acting on the valve needle and to do so at an acceptable energy cost.
Because of the major problems with direct electromagnetic actuation of the valve needle, precontrolled systems have been proposed as generally disclosed in Federal Republic of Germany publications DE OS 2914966 and DE OS 2927440. In such arrangements the injection nozzle is provided with an additional injection piston, which is located directly in the nozzle and is actuated through a hydraulic transmission at a relatively low pressure of about 100-300 bar. Because of the hydraulic transmission, the required volume flow in the fuel inlet line is increased by the factor of the transmission ratio. The fuel is drawn from the inlet line intermittently. The intermittent inflow, in turn, causes pressure oscillations the amplitude of which depends almost exclusively on the inflow speed of the fuel. Therefore, the amplitude of the pressure oscillations is increased as compared with a directly operated injection valve at equal fuel line cross-section by the factor of the transmission ratio and, at the same time, because of the lower system pressure, the relative amplitude likewise increases by the factor of the transmission ratio. At equal fuel line cross-section and a normal transmission ratio of about 5, therefore, the amplitude of the pressure oscillations referred to the system pressure is increased by a factor of 25. These pressure oscillations can be absorbed only in part with accumulators disposed directly in the valve. But the main disadvantage compared with directly controlled injection valves is the high additional cost of construction.
An injection valve with directly actuated valve needle is disclosed in Federal Republic of Germany publication DE OS 2949393. There the electromagnet has a helical armature with several simultaneously excited magnet coils. To reduce chatter, the magnet has two braced telescoped cone elements in which the kinetic energy is consumed toward the end of the valve closing process by mechanical friction.
The special geometric form results in a thin-walled, low eddy current magnetic circuit with a light armature which permits rapid actuation at high magnetic force. Furthermore, because of the elongated armature, largely free of lateral forces, a reliable suspension results. In order to obtain sufficiently rapid setting movements with this electromagnet the bulk of the magnetic field energy must by supplied during the setting process in a very short time. To this end, an enormous electric power must be made available in a short time. In static operation the electric energy consumption is increased, as compared with magnetic circuits with only one coil, by the number of magnet coils. This is attributable to the fact that the electric excitation required for a given induction depends essentially only on the air gap length and not on the surface of the working air gap. On the whole, the magnetic circuit requires a high manufacturing cost. Winding of the core is complicated, and the multiple air gaps require very close machining tolerances. Further, the wear properties of the damping cones appear to be critical.
To simplify the manufacture of the electromagnet and to improve the efficiency of the electric energy conversion, the use of electromagnets with only one coil is appropriate, provided sufficiently high setting forces combined with sufficient leakage field and eddy current depletion can be achieved with them. Because of their simple mechanical design, cylindrically-symmetrical forms are favorable. The known electromagnetic injection valves with one magnet coil always have a closed electromagnetic circuit of solid, low-retentivity material of high permeability, with one or more air gaps active in pull-up direction, in which is formed the predominant part of the mechanical force that causes the armature movement. These air gaps may be referred to as working air gaps. To avoid sticking of the armature due to residual magnetic forces in the pulled-up or pulled-in state, the magnetic circuit is, as a rule, designed so that a small air gap remains when in the pulled-up or pulled-in state. This air gap is obtained by mechanical limitation of the armature stroke or also by providing a radial air gap directly around the armature. These remaining air gaps may be referred to in the following as residual air gaps. A similar effect can be achieved also by coating the armature and the magnet poles with thin, non-magnetizable films, which at the same time improve wear resistance and corrosion stability.
It is known that between smooth surfaces hydraulic adhesion forces result. To reduce the hydraulic adhesion and to improve the wear properties, a roughening of about 0.5 micrometers of the joint surface of the core or of the armature is recommended as in Federal Republic of Germany publication DE OS 3013694. One of the two joint surfaces should be made as smooth as possible.
It is generally believed that the pole cross-section should always be narrowed or at least not increased in the region of the working air gaps. By such means one always obtains the saturation induction of the magnet material in the region of the working air gaps with the armature pulled-up or pulled-in. As the mechanical force increases quadratically with the air gap induction, the maximum possible magnetic force is reached by saturation of the poles at a given pole cross-section.
To achieve high metering precision, rapid and low-bounce movement processes of the armature are required. The bounce can be considerably reduced by a supplementary mass disposed between armature and reset spring, the movement of the armature and supplementary mass being matched by appropriate selection of the mass and force conditions in such a way that toward the end of the first bounce cycle the movement of armature and supplementary mass occurs counter-directionally, and thereby the kinetic energy of the armature is to a large extent dissipated. Further, when using a suddenly changing spring characteristics in conjunction with the supplementary mass system, low-bounce movement processes with extremely short reset times are obtained. However, it is believed that the technological realization of such a characteristic in electromagnetic injection valves presents considerable technical difficulties because of the extremely small armature stroke and for this reason, very steep linear spring characteristics probably should be preferred.
The efficiency of the electric energy conversion is greatly impaired by leakage field lines, which do not go through the working air gap, and by eddy currents. The eddy currents can be greatly reduced by the use of thin-walled magnetic circuits. The degree of efficiency reduction by the leakage field is influenced most strongly by the geometric arrangement of the air gaps.
An electromagnetic injection valve with thin-walled magnetic circuit and flat armature have been described as in United Kingdom publication GB PS 14 59 598 and European Patent Office publication EP-OS 0 054 107. Electromagnetic injection valves with flat armature have a critical, poorly reproducible setting behavior because of deficient armature suspension. The efficiency of the electric energy conversion is low because in the dropped or released state these magnetic circuits have a strong leakage field because of the double working air gap and because of the unfavorable position of the air gaps below the coil.
By the use of thin-walled magnetic circuits with a bowl or cup-shaped armature the armature suspension and the electromagnetic efficiency can be improved substantially over flat armature magnetic circuits. For relatively large electromagnets the armature suspension is affected by a thin-walled guide tube. Although thereby the leakage field is substantially reduced as compared with flat armature magnets, there still is a considerable leakage field in particular in small electromagnets with relatively large armature strokes.
It is often believed that in magnetic circuits with a double working air gap the pull-up or pull-in speed is considerably reduced. (Example: flat armature magnet.) By comparison with a magnetic circuit with single working air gap (example: plunger magnet), here, at equal total pole cross-section and therefore equal maximum force, the working air gap length is doubled, and the pole surfaces are cut in half, whereby the inductance of the magnetic circuit at equal coil data is reduced to one fourth and the rate of exciting current rise is quadrupled.
Especially small armature masses are obtained with injection valve assemblies with spherical armatures. The spherical armature is usually disposed below the magnet coil. However, these injection valves have high leakage factors. The poles of these valves are either flat or conical. In the known injection valves with conical pole the attachment of the core is on the side opposite the pole, which leads to centering problems. It is now proposed as an improvement over the prior art to compose the magnetic circuit in part of thin metal sheets and to install it in a housing of non-magnetic material in order to reduce the eddy current losses.
In the injection valve shown in European Patent Office publication EP-OS 0 007 724, the spherical armature is reset by hydraulic forces, so that an additional reset spring is not necessary. The injection valve has a central bore with radially arranged slots. The inflow to the injection nozzle is partially closed by the spherical armature toward the end of the pull-up or pull-in process, so that through the throttling between the inflow bore and the rest of the space around the spherical armature a differential pressure results which creates the closing force. The magnetic circuit of this valve has a very large radially disposed residual air gap, in which the spherical armature is to be centered by hydraulic forces. In view of the fact that due to hydrodynamic oscillation processes stable stationary flow conditions do not prevail until a considerable length of time after the start of the opening movement, and because of strong radial magnetic interference forces occurring at the least of eccentricities, the stability and reproducibility of the armature movement appears doubtful.
To improve the atomization, it is customary for induction passage injections for Otto cycle engines to surround the fuel issuing from the injection nozzle with a secondary air stream. The secondary air stream is branched off behind the intake air filter of the internal combustion engine. The injection valve is disposed behind the throttle valve of the engine, so that the pressure gradient at the throttle valve is available to generate the secondary air stream. In prior art injection valves the secondary air stream has approximately the same temperature as the secondary air stream drawn in by the engine.
The prior art electromagnetic injection valves have inferior electromagnetic efficiencies. Nevertheless, in order to attain sufficiently rapid setting movements at low energy consumption, one uses, as a rule, special electronic actuating circuits which, during the pull-up or pull-in process excite the electromagnet with a powerful current surge, thereafter lowering the coil current by gating to the much smaller holding current. Excitation is effected directly with the respective onboard power supply voltage. For special requirements with respect to dynamics, a pre-excitation may be effected before the actual setting process. These actuating circuits are complicated and create additional costs. Prior art patent literature also disclose circuits for the actuation of electromagnets where a rapid excitation is achieved by capacitor discharge. Such circuits have not heretofore been used in electromagnetic injection valves.
Generally, the movement cycle of a conventional prior art electromagnetic injection valve can be divided into four main phases.
During the first phase after application of the exciting current no armature movement takes place. This phase is referred to in the following as pull-up or pull-in delay. The armature movement begins as soon as the magnetic force exceeds the mechanical counter-force. The length of time between the start of the armature movement and arrival in the end position of travel of the armature is termed pull-up or pull-in time. In the usual injection valves, the armature is firmly connected with the valve needle, therefore the valve needle executes the same movement as the armature. After disconnection of the exciting current, the reset movement of the armature is delayed by eddy currents and the electric damping of the coil and this time is called reset delay. The reset movement of the armature begins with the moment in which the mechanical reset forces exceed the magnetic force. The time during which the armature moved back into the inoperative position is referred to as reset time.
In electromagnetic injection valves, the effect of the coil resistance on the magnetic force buildup can be neglected at least at the beginning of the excitation. The magnetic force buildup is then independent of the armature movement. The magnetic force increases quadratically with the time. Because of the slow force buildup, little excess of force is available at the beginning of the pull-up or pull-in movement for the acceleration of the armature, so that, depending on the reset spring force, the stroke begins much more slowly still approximately with the third to fourth power of the time. It therefore generally takes the armature up to 75% of the pull-up or pull-in time to travel the first third of its stroke.
High-pressure injection valves require, at the beginning of the valve needle movement, a very high force to overcome the hydrostatic force which presses the valve needle onto the needle seat. This force, however, drops off very steeply at the start of the needle movement, since at a very short stroke a partial pressure equalization under the seat surface of the needle takes place, which, in turn, greatly reduces the hydrostatic force. Therefore, the force required for raising the valve needle decreases rapidly with increasing stroke to about 10 to 20 per cent of the opening force.
For the actuation of the valve needle in the usual electromagnetic high-pressure injection valves extremely strong electromagnets are required, the maximum magnetic force of which considerably exceeds the opening force of the needle and of the reset spring, so as to bring about sufficiently rapid setting processes. Toward the end of the pull-up or pull-in process, an extremely high excess of the magnetic force over the mechanical actuating force results, so that only a small portion of the magnet work serves to overcome the mechanical counter-force. Because of the very large excess of magnetic force, the reset delay is long. Even in case of pre-excitation of the magnet coil, the major part of the electric energy must be supplied during the brief pull-in process, so that when operating with the usual on-board power supply voltage of 12 volts the required peak currents may readily exceed values of 100 amperes.
The injection valve according to the invention, on the contrary, utilizes the kinetic energy of the armature to overcome the high hydraulic setting forces. To this end the armature is arranged so that it impinges on the valve needle at a relatively high speed only after having traveled about 30% of the armature stroke. Such an arrangement offers a number of advantages.
Firstly, with such an arrangement it is not necessary that the maximum magnetic force exceed the maximum hydraulic setting force, so that very small electromagnets with small armature mass can be used. Secondly, the movement time of the valve needle is much shorter than the pull-in time of the armature, so that already at a relatively slow excitation of the magnet coil a sufficiently rapid setting process is obtained. The work capacity of the electromagnet is utilized almost completely. The setting movement begins with a high initial speed, owing to which the pressure conversion occurs almost without delay in the nozzle holes, and therefore a high atomization of fuel is achieved immediately after start of injection. Typically the opening time is about 0.2 ms., unequalled until now in electromagnetic injection valves according to the invention. Despite the short setting times, the motion is soft and well reproducible with a low impingement speed toward the end of the setting movements, whereby the mechanical load on the structural parts and the wear properties are improved.
To obtain short reset times, there should be used in the area of the opening stroke of the valve needle a spring arrangement with supplementary mass and suddenly changing spring characteristic. That is, a supplementary mass is disposed between the armature and reset spring in such a way that after impingement on the armature the supplementary mass effectively detaches, relieving the armature of the reset spring force, so that upon rebounce of the armature a high excess of magnetic force is available for decelerating the bounce movement. The system is matched so that the then following collision of armature and supplementary mass is counter-directional, so that the kinetic energy of the armature is thereby dissipated to a large extend. Here, however, it was still believed that more stable movement conditions would be obtained with very steep linear spring characteristics than with suddenly changing spring characteristics. It has also now been discovered that it is possible to have systems with suddenly changing spring characteristic which have very stable movement conditions and are extremely insensitive to minor manufacturing imprecisions or respectively to possible wear. That is, a change of about 10% of the range of action of the strong spring causes, for example, in the range of the technically meaningful dimensions, only a variation of the setting time of about 2%. Because of the simpler manufacture, therefore, suddenly changing spring characteristics should always be preferred over steep linear ones.
Because of the high reset spring force, which is only just below the maximum magnetic force, the following resetting process occurs almost without delay with vary high initial acceleration. After impingement of the valve needle on the needle seat, the armature detaches and continues its travel with almost undiminished speed, so that a very high excess of force is available for reducing the otherwise prior art armature bouncing.
Furthermore, with a suddenly changing spring characteristic the reproducibility of the individual injections can be improved. Concerning this, consider first the disturbing influence of a fluctuating actuating voltage.
The injected quantity as a function of the duration of an electric actuating signal is composed of two parts, namely, the injected quantity during the transitional phases and a stationary portion. The stationary portion is adjusted, as a rule, by varying the valve needle stroke, whereby the flow through the injection valve is varied. The non-stationary portion of the injection quantity depends to a large extent on the dynamics of the injection valve, which can be acted upon by varying the reset spring force. Variation of the reset spring force affects, in the usual injection valves with single reset spring, both the pull-up process and the reset process. With increasing reset spring force the total pull-up time increases and the total reset time decreases. As the two effects are oppositely directed with respect to the injected quantity, wide dispersions of the reset spring force will result among the individual injection valves. Because of the wide scatter of the reset spring force, identical injection quantities will result for the individual valves only at a certain exciting voltage at which the calibration is carried out. At deviating exciting voltages, a scatter of the injection quantities results among the individual injection valve assemblies, which, of course, is undesirable.
Much more favorable conditions result with the injection valves with suddenly changing spring characteristic as proposed by Applicant. In such a proposed arrangement, only an adjustment of the high spring force with the armature pulled up is effected, while the low spring force at the beginning of the pull-up process remains almost uninfluenced. For the dynamics of the pull-up process, however, the spring force at the beginning of the pull-up process is almost exclusively determining. Therefore, only the drop-off process is notable influenced in the calibration, so that even at deviating exciting voltages uniform variations of the injection quantity result for all valves and are accordingly taken into consideration by the electronic actuating circuit.
Heretofore, it was generally believed that to reduce chatter there should be a firm, inflexible abutment. However, the chatter can be further reduced by making the abutment flexible. In this connection it is necessary, however, that by appropriate design of the abutment the natural frequency of the abutment is placed into a region where the rebounce movement of the valve needle and the movement of the abutment are counter-directional otherwise the chatter will be increased. With a flexible abutment, moreover, the mechanical shock at the moment of collision and therefore the wear are greatly reduced.
With the dynamic arrangement as herein proposed, where the maximum hydraulic force exceeds the maximum magnetic force, the valve needle should be pressurized by the system pressure on all sides. It is of course, possible also to seal the top and bottom of the valve needle from each other by a narrow guideway and to compensate the hydrostatic force remaining when the valve needle is open with a helical spring; then, however, at varying system pressure greatly varying setting forces will result which impair the reproducibility of the setting movement. In addition, the sealing of the valve needle requires extremely high precision and should the action of the helical spring force be eccentric the valve needle will be exposed to strong disturbing forces.
If the moving parts are exposed to the full system pressure, a special design of the various function surfaces is necessary in particular for high-pressure injection valves. In fact, when two smooth surfaces lie one on the other, the fuel film between these parts is displaced, and is removed from the action of the ambient pressure, so that, especially at high ambient pressures, the parts are firmly pressed together. This phenomenon is referred to in the following as hydraulic sticking. If the individual abutments of the valve system were given smooth surfaces, the parts would adhere firmly to each other after only a single actuation so that further operation would not be possible.
Closer study of the hydraulic processes in the moving gaps has shown that the gap flow can be divided into several phases.
In the first phase of the movement, as the gap closes, almost exclusively acceleration forces are active in the flow. Compared with the other forces, the amount of the mechanical reaction force is negligibly small.
As the gap continues to close, increasing energy loss occurs due to the kinetic energy of the outflowing liquid. This kinetic energy is almost completely whirled up and brings about a perceptible damping of the setting movement. The mechanical reaction force increases quadratically with the setting speed and also quadratically with the reciprocal value of the gap width. For annular gaps the reaction force increases with the third power of the gap width and for round surfaces even with the forth power of the diameter.
If the gap is very narrow, the friction forces finally predominate in the flow. They increase linearly with the setting speed and with the third power of the reciprocal value of the gap width. Toward the end of the movement, the friction resistance in the liquid is very great because of the narrow gap, so that removal of liquid is greatly hindered. Unless the movement speed has been greatly diminished by the preceding damping, there results an exceedingly strong pressure increase in the liquid between the gaps bringing about in conjunction with the compressibility of the liquid an almost loss-free movement reversal. This is hereinafter referred to as the liquid cushioning phase. In this phase, pressures up to several 1000 bar may occur even in low-pressure injection valves.
After the movement reversal, the gap volume increases. With parallel smooth gaps not enough liquid can follow from outside so that the flow is interrupted. Due to the then existing pressure decrease the air dissolved in the fuel is eliminated and cavitation phenomina occur.
By a geometric configuration of the gaps which permits a sufficient supply of liquid, the hydraulic sticking and interruption of the flow can be prevented.
In the evaluation of the hydraulic gap processes, the respective "Navier-Stokes equations" lead to complicated non-linear differential equations whose evaluation is possible only with numerical methods. Exact dimensioning rules can therefore be stated only for a specific case.
Generally, hydraulically favorable conditions result when one of the two abutting surfaces is ground in flow direction from the inside out with a surface roughness of about 1-5 micrometers, while the other is made very smooth for example by lapping. The carrying share of the ground surface should not exceed 10%. To reduce wear, both abutment surfaces are hardened, preferably by nitriding. The abrasion gaps in flow direction also permit the removal of any small particles breaking out, so that the further flow of liquid is not hindered.
Another possibility for preventing hydraulic sticking consists in that one of the two abutment surfaces is formed in collar form, dish form, or membrane form with little cushioning capacity and rests on the other abutment surface in ring form when the gap is closed. As the mechanical force changes, the parts can detach and roll off on each other first at the edge and then progressively farther inward in flow direction, so that a largely unhindered supply of liquid into the gap is possible. The interaction of the parts can be further improved by a slight barrel shape of one of the two abutment surfaces. If the abutment surfaces are sprung, the natural frequency of the abutting parts should, as has been described, be matched in such a way that a counter-directional collision results.
Further, one of the two abutting surfaces may be beveled, so that the gap cross-section increases from the center outwardly. The angle of the bevel preferably should not exceed 1.degree. and should usually be even much less. For gaps with very large surfaces, a strong damping can thereby be achieved toward the end of the setting movements, largely suppressing the always existing chatter.
The remaining hydraulic effect on the movement of the individual parts of the injection valve are quite minor, provided sufficient cross sections for pressure compensation exist. This is attributable to the fact that any pressure disturbances are compensated at the speed of sound in the fuel. By contrast, the maximum movement speeds of the individual parts, about 1-2 microseconds, are very low, so that in the evaluation of the hydraulic effects on the movement conditions, with the exception of the gap processes, a hydrostatic approach is sufficient.
Nevertheless, strong hydrodynamic oscillations may, of course, occur, but they have little influence on the movement of the individual structural parts. Such oscillations can be employed for controlled influence on the injection process. Care must be taken, however, that these oscillations occur only at the injection nozzle itself and are not coupled into the connecting lines between the individual injection valves, in order to stabilize the system pressure before the individual injection valves and not to impair the reproducibility of the individual injection processes. This is appropriately achieved by disposing compressible elements in direct vicinity of the injection nozzles or respectively the valve member. As the amplitude of the pressure oscillations depends directly on the flow velocity of the fuel, the inflow cross-sections to the individual injection valves should be taken as large as possible.
In low-pressure injection valves for induction passage injection, the atomization quality can be improved in known manner by supplying atomization air. In the known injection valves, the atomization air is branched off behind the intake air filter of the engine. The injection valve is disposed behind the throttle valve of the engine, so that flow of the atomization air is brought about by the pressure difference resulting at the throttle valve. With the throttle fully open, however, there is no longer any appreciable pressure difference, so that the flow of the atomization air almost ceases.
On the other hand, however, with the throttle open, a strong engine intake air stream exists which leads to a perceptible pressure drop at the intake air filter of the engine. Owing to this, there exists in the induction passage of the engine, at least during considerable time portions of the respective cycle, a sufficient vacuum relative to the ambient air, which can be utilized to create high atomization air speeds. As an example, already at a vacuum of 50 mbar there results an air flow velocity of about 100 m/s--a value at which a very good improvement of the atomization is achieved.
Utilization of the induction passage vacuum is possible also with the throttle fully open if the atomization air is taken from a separate air filter which serves exclusively for the filtering of the atomization air. This measure is especially effective because low induction passage vacuums are linked with high combustion air velocities and therefore with great throttling at the intake air filter, whereas the throttling at the atomization air filter decreases because of the decreasing atomization air speed. An especially simple and effective design results if the separate atomization air filter is disposed directly at the injection valve and the atomization air is guided through the coil space of the injection valve, so that at the same time improved coil cooling is achieved.
An additional great improvement of the atomization and of the engine efficiency can be achieved by heating the atomization air. To this end, a heat exchanger, which may consist, for example, of a spiral tube, is disposed directly in the hot engine exhaust gas stream. The heat exchanger is placed between the air filter and the atomization device. Thus, with the throttle closed, almost exclusively high-temperature atomization air is supplied to the engine as combustion air. Thereby the fuel is excellently nebulized and precipitation of fuel on the induction passage walls is reduced. The high intake air temperatures reduce the ignition delay in the partial-load range and thereby improve the efficiency of the engine. The improved combustion process permits expansion of the lean range of the engine and reduces pollutant emission. With increasing opening of the throttle, the hot atomization air stream is increasingly mixed with cold air, so that the temperature of the combustion air decreases. In this way a sufficient margin from the knock limit of the engine is ensured. With the throttle fully open, the heating of the combustion air is now insignificant because of the small proportion of atomization air, although here, too, a great improvement of the atomization is achieved because of the high temperature of the atomization air. Furthermore the flow velocity of the atomization air is greatly increased by air-heating especially at low pressure differences, since increasing air temperature at equal pressure difference always brings about a strong increase in flow velocity. Furthermore, because of the good adaptation of the mixture preparation to the requirements of the engine characteristics, a considerably smaller adjustment range of the ignition is required.
The heating of the atomization air may, however, lead to considerable problems with the injection due to vapor bubble formation of the fuel in the injection valve. To prevent vapor bubble formation, therefore, heat insulation of the injection valve from the atomization device and additional cooling of the injection valve by flushing with fresh fuel is preferable.
At high loads, the performance of Otto cycle engines is limited by engine knocking setting in. In modern engines this is prevented by throttling back the pre-ignition as a function of the signals of a knock sensor. With the pre-ignition throttled back, the engine efficiency is reduced. At high engine loads the efficiency can be improved by water injection. Thereby the combustion peak temperatures are greatly reduced without leading to a reduction of the efficiency of the motor combustion. From the lower peak temperatures a considerable decrease in nitric oxide is to be expected. It is, as a rule, not necessary to throttle back the pre-ignition and usually it can be further increased. Excellent adaptation to the engine characteristics is possible by injection of water at low pressure into the induction passage of the engine through an electromagnetic injection valve as a function of a knock sensor signal. This measure reduces the water consumption. And since water is fed only at high loads and therefore at high engine temperatures, condensation of the water in the engine and therefore increased corrosion need not be feared. No special requirements need be set for the atomization quality, as the water reaches the engine only in relatively thick drops anyway, and evaporation takes place only toward the end of the compression process and during the combustion process. Suitable for the supply of water are therefore also simple water carburetors (gasifiers) which consist only of a main nozzle system and float chamber, and in which the supply of water is controlled by a simple solenoid dependent on the engine ignition knocking.
In experiments it has now been found that with water injection the combustion occurs almost without residue and the deposition of combustion residues in the engine is almost completely prevented. Furthermore, when using the described system also in Otto cycle engines nearly any degree of supercharging is possible, limited practically only by the mechanical strength of the engine. The injection of water takes place in supercharging always before the supercharger, to achieve an additional improvement of the atomization by mechanical forces and an improvement of the supercharger efficiency.
The injection of water permits, also for conventional Otto cycle engines, the use of fuels with a very low octane number, without having to throttle-back the compression ratio of the engine. An especially good adaptation to the engine characteristics is achieved with the injection of water in conjunction with the previously described hot air atomization.
To obtain reproducible fuel injection quantities, calibration of the injection valves is always necessary. The calibration of the injection valves is normally done with fuel. The manufacture of low-pressure injection valves is done with air with respect to the stationary component of the fuel flow similar conditions are obtained if the Reynolds Numbers of air flow and fuel flow are in agreement. Furthermore, the air velocities must be considerably lower than the velocity of sound if air, in order to obtain comparable conditions. The differential pressure for the creation of the air flow may therefore be only some 10 mbar. Now, however, the kinematic viscosity of air in the ambient state is much greater than that of fuel. The kinematic viscosity of air can be reduced by a pressure increase. Generally an air pressure of 5-10 bar is sufficient, which this is considerably higher than the usual fuel injection pressure of about 0.7 to 3 bar.
The valve setting processes are generally greatly influenced by hydrostatic forces. The calibration of the dynamic behavior and hence of the non-stationary component of the fuel injection quantity occurs, therefore, at an air pressure which corresponds to the fuel injection pressure. This, of course, does not take into consideration the damping of the setting movements by the fuel and the effect of the hydrodynamic oscillations; however, the end points of the respective setting movements, which most influence the non-stationary components of the injection quantity, are well reproducible. Any deviations can be taken into account in this calibration method by appropriate correction factors. Measurement of the movement process of the armature can be effected, for example, by photo-cells or by evaluation of the electro-dynamic voltage reaction in the magnet coil.
In the proposed injection valve, which utilizes the kinetic energy of the armature to overcome the opening force, sufficiently short setting times can be obtained even at relatively long armature pull-up or pull-in times. This requires minor flux increase rates in the magnetic circuit. At low flux increase rates, the eddy current formation is also greatly reduced, thus making it possible to use relatively thick-walled magnetic circuits. Because of the greatly reduced losses, the maximum power requirement is lowered by about one order of ten as compared with the usual design.
In the ideal case, the magnetic force buildup is, at equal initial inductance, independent of whether the electromagnet has a single or a double working air gap. In the ideal case, the magnetic force depends only on the energy stored in the magnetic field and on the armature stroke. The electric energy consumed in a given period of time, neglecting the coil resistance, depends only on the initial inductance of the electromagnet.
In electromagnets with a double working air gap, the number of turns of the exciting coil must be quadrupled in order to obtain the same inductance as with a magnet with single air gap, so that at equal current path and equal current density the winding cross-section must also be quadrupled. Furthermore the cross-section of the poles is cut in half and the total air gap length is doubled. This makes the reluctance of the magnetic circuit and hence the leakage field of the electromagnet such greater. As the magnetic force decreases quadratically with the leakage factor, the leakage field is of special importance for the dynamic behavior. The leakage field increases the inductance of the coil and greatly reduces the magnetic force in the saturation range with the armature dropped.
On the other hand, in electromagnets with double working air gap the eddy current formation is reduced to about one fourth because of the halved wall thickness of the magnetic circuit. To achieve a sufficient eddy current depletion, the wall thickness of the magnetic circuit preferably should not exceed 0.5-1 mm.
With such small wall thicknesses, however, at the usual injection valve dimensions and with the armature dropped, the magnetic resistance of the air gaps is considerably greater than the resistance between core and yoke, so that a strong leakage field forms, which by-passes the air gaps. In low-pressure injection valves with flat armature magnet, for instance, the leakage field flux may, at the usual magnetic circuit dimensions, amount to as much as 75% of the total flux, so that the efficiency of the electromagnetic energy conversion decreases in the same proportion. As the dynamic behavior of the electromagnet is determined mostly by the speed of field buildup at the beginning of the pull-up movement, it is especially important to reduce the leakage field to obtain rapid, low-loss setting movements.
Favorable efficiencies are attainable only with special pole arrangements. At small pole cross-sections, electromagnets with one working air gap are favorable because of the reduced reluctance. The working air gap should preferably be placed approximately in the center of the coil, since at this point a flux concentration is located which permits a low-loss energy conversion. In electromagnets with double working air gap the best efficiency results with a bowl-shaped armature which embraces the coil and whose poles are arranged so that they each cover about one fourth of the coil. In the case of elongated coils, the leakage field is then reduced by about 75% as compared with a flat armature magnet with equal pole cross-section and equal coil dimensions. With the pole arrangement an equally good efficiency is obtained as with an electromagnet with single working air gap in the center of the coil, but with halved magnetic circuit cross-section and therefore greatly reduced eddy current losses at equal magnetic force.
In electromagnets with bowl-shaped armature, however, the sealing and anchoring of the magnet coil is difficult. In this respect, magnetic circuits with double working air gap are favorable where the outer pole of the armature is formed by a collar of small diameter. Such electromagnets are described in Federal Republic of Germany publication DE OS 3149916 and European Patent Office publication EP OS 0076459. Both electromagnets have a short armature, the poles of which are located below the coil and therefore have strong leakage fields. In particular for the electromagnetic injection valve described in said DE OS 3149916 it would seem that because of the relatively thick-walled magnetic circuit hardly any improvement over the known injection valves with single working air gap will result. One advantage of this design, however, is the almost lateral force-free magnetic force buildup even in case of possible slight eccentricities of the armature suspension.
Considerably better efficiencies are obtained with such electromagnets if the inner pole is arranged above the coil center. The highest magnetic force is obtained when the pole cross-sections are approximately the same, and the inner pole is arranged approximately at the level of the upper fourth of the magnet coil. For low-pressure injection valves with appropriate dimensions often only small magnetic forces are required, which can then be supplied with a single working air gap and a wall thickness of the magnetic circuit of about 0.5 mm. Here, too, a double working air gap is favorable in order to achieve a lateral force-free armature suspension. To this end the pole cross-section of the outer pole can then be greatly increased, to reduce the reluctance of the respective air gap and hence the leakage field. With such a layout the best efficiency is obtained if the inner pole is arranged approximately in the center of the coil.
At very low mechanical counter-forces and small armature mass, low magnetic forces are required. In low-pressure injection valves, therefore, the pole cross-section, in contrast to the usual dimensional designs, can even be enlarged as compared with the magnetic circuit cross-section, in order to reduce the reluctance of the air gaps and hence the leakage field with the eddy current losses being reduced at the same time. With such an arrangement, an almost loss-free energy conversion is obtained. The reduced reluctance permits, at equal thermal load and equal inductance as in a conventional electromagnet, the use of much smaller magnet coils with a small number of turns. On the other hand, with the usual dimensioning, where in the interest of a low leakage field the pole cross-section is not greater than the rest of the magnetic circuit cross-section, high saturation induction forces result which far exceed the mechanical counter-force and thus lead to a long reset delay. The reset delay must then be reduced by an electronic holding current reduction. By contrast, the herein proposed arrangement permits using simple actuating circuits without holding current reduction with the dynamics being improved at the same time. In addition, the proposed arrangement permits, in a simple manner, the improvement of electromagnetic injection valves already in production in that the core cross-section is reduced above the pole in an essential part as by drilling open.
Another major leakage field reduction can be achieved at the usual magnetic circuit dimensions in particular for small electromagnetic injection valves by providing the housing means of the rotationally-symmetrical, all-enclosed magnetic circuit with large-area openings. Thereby the reluctance between housing and core is increased, so that the strength of the leakage field is reduced.
In high-pressure injection valves, the creation of sufficient magnetic forces requires large pole cross-sections, the air gap having only a low reluctance. With the above measures the leakage field can be further reduced, so that sufficiently high electromagnetic efficiencies result also with materials of very low permeability. This permits the use of powder composite materials, where a low-retentivity powder is embedded in an insulating plastic. These materials have a high electric resistance, so that the formation of eddy currents is prevented almost completely. In general, however, the maximum relative permeability cannot exceed values of 200-300. With such materials compact magnetic circuits can be constructed, which have a sufficient mechanical strength and can withstand high pressures. For reliable suspension the armature is connected as with a long thin-walled guide tube, which serves at the same time as abutment, so as not to expose the mechanically relatively soft magnet material to impermissible stresses. The armature can be made by integral pressing with the guide tube in one operation. For increased magnetic flux the thin-walled guide tube may consist of low-retentivity material, which is surface-hardened as by nitriding to improve the wear properties. In this hardening process the low-retentivity properties are reduced only little. When using sufficiently pressure-resistant coils, the magnet material can be pressed directly around the coil, to facilitate the sealing and to simplify the manufacture.
For high-pressure injection valves the usual wire coils are not very suitable. Here only few turns and hence only few courses of turns are required to obtain sufficiently low inductances. Between the ends of the individual courses high induction peaks will occur, in particular upon switching off, which endanger the insulation of the coil. Much more favorable is the use of foil coils, which permit a much higher mechanical as well as electrical stress. Produced in quantity, such foil coils are also less expensive than wire coils. The coil may consist for example of oxidized aluminum foil, so that an insulating intermediate layer may be dispensed with. Also a coil former may be dispensed with, so that also a better utilization of the winding space is obtain. To improve the mechanical strength, the coil is preferably impregnated with plastic under vacuum. Contacting can be effected, for example, through metal sleeves slit lengthwise, to further improve the mechanical strength. Another possibility is to fold the foil ends over and to bring them out at right angles to the winding direction. At sufficient coil strength it is favorable to clad the coil directly, possibly in several operations, with powder composite material.
If the coil space is sealed, the use of ceramic coil formers is favorable. As material the newly developed high-strength ceramic materials known from engine and turbine construction are preferably used. To improve the load capacity of the coil former, the coil should be wound at highest possible traction, to obtain a mechanical pre-stress of the coil former.